A phenol resin is a material excellent in the balance between heat resistance, dynamical performance and electric characteristics and the cost, and utilized in various industrial fields. In particular, applicability to various fields has recently been found as to a granular or powdery phenol resin possessing thermofusibility and a thermosetting property, and some products are already put on the market as multiuse materials. For example, Japanese Patent Laying-Open No. 58-111822 (Patent Document 1) discloses a granular or powdery thermosetting phenol resin consisting of a condensate of a phenolic compound and formaldehyde, and this is put on the market with a trade name “Bellpearl (registered trademark) Type S” (by Air Water Inc.).
In general, the aforementioned thermosetting phenol resin powder is employed singly or in a state mixed with another material to be shaped and hardened. Shaping denotes an operation of heating, melting and compression-molding a solid (powdery, for example) thermosetting phenol resin or molding a thermosetting phenol resin dissolved in an organic solvent to be liquefied by an operation such as cast molding or impregnation. However, shaping employing a liquefied phenol resin containing an organic solvent had a problem of influence on safety and the environment resulting from the organic solvent.
In such a context, a technique of not liquefying a thermosetting phenol resin with an organic solvent but dispersing a thermosetting phenol resin powder in water for liquefying the same and employing this as a raw material for shaping has recently been proposed. According to such a water dispersion liquid, the influence exerted on safety and the environment due to the organic solvent is avoided.
When powder-mixing a thermosetting phenol resin powder with another material and shaping this, it is required that the mixing with another material is homogeneously performed in a short time, and it is required for this purpose that the thermosetting phenol resin powder can be impregnated into fine clearances of the other material with low resistance. When preparing a water dispersion liquid from the thermosetting phenol resin powder and shaping this, it is required that the same can be rapidly and homogeneously dispersed in water and the water dispersion liquid has low viscosity, i.e., is easily flowable also in a high concentration region. When shaping a composite of this water dispersion liquid and another material, further, it is required that the water dispersion liquid can be so easily impregnated into clearances of the other material that mixing is homogeneously and rapidly performed.
In order to satisfy the aforementioned requirements, the shapes and modes of particles of the thermosetting phenol resin must be properly controlled. In other words, it is required that (i) the average particle diameter of the particles is sufficiently small, and (ii) there is hardly any secondary aggregate resulting from aggregation of primary particles, in order to attain easiness in mixing with the other material and low viscosity of the water dispersion liquid. When also taking account of dispersibility in water and low viscosity of the water dispersion liquid, it is more desirable that (iii) the particle size distribution of the particles is sufficiently narrow, and/or (iv) the shapes of the particles are closer to a spherical shape, in addition to the above (i) and (ii). The aforementioned sufficiently small particle diameter must be at least not more than 20 μm, more preferably not more than 10 μm, in consideration of application of the phenol resin powder or a hardened substance thereof to various industrial uses.
Further, while a resin having low melt viscosity or low solution viscosity has been generally preferably used as a thermosetting phenol resin due to such advantages that a molding pressure can be reduced and a dense and excellent mixed state can be formed with another material, for example, such a low-viscosity thermosetting phenol resin generally contained large quantities of a highly toxic monomer phenol and a low condensation component, and had a problem in safety and environment. Therefore, development of a thermosetting phenol resin containing no such monomer phenol etc. or having an extremely small content of monomer phenol etc. has been strongly awaited.
However, although a large number of studies have been heretofore conducted as to the thermosetting phenol resin powder, it is the present situation that a thermosetting phenol resin powder having the aforementioned characteristics is not yet known and a production method suitable for mass production of such a thermosetting phenol resin powder is not yet known either.
For example, the aforementioned Patent Document 1 describes a technique of obtaining a granular or powdery thermosetting phenol resin by optimizing synthetic conditions such as the ratios of quantities of formaldehyde, phenol, hydrochloric acid and a water medium as used and a temperature condition, and the obtained thermosetting phenol resin contains neither a harmful phenol monomer nor a low-molecular condensation component, and has high safety. However, the thermosetting phenol resin had such points to be improved that (i) the primary particle diameters are relatively large, (ii) the quantity of secondary aggregates formed by aggregation of the primary particles is relatively large, (iii) the particle size distribution is wide, and (iv) the resin contains a large quantity of particles having shapes other than a spherical shape.
Japanese Patent Laying-Open No. 2-167327 (Patent Document 2) describes a technique of producing a pulverized phenol resin by dissolving a phenol resin in an organic solvent, thereafter adding a protective colloid solution consisting of a catalyst, a hydrophilic polymer compound and an aldehyde thereto and performing reaction. However, the phenol resin obtained by this method has a relatively large average particle diameter of 50 to 500 μm (refer to Example). Further, the production method has a problem in safety and environment in the point that the organic solvent is employed.
Japanese Patent Laying-Open No. 50-98537 (Patent Document 3) describes a technique of obtaining an unhardened phenol resin powder by adding a cellulosic compound to an initial condensate obtained by reacting a phenolic compound and a formaldehyde in the presence of at least either an acidic catalyst or a basic catalyst and a catalyst of a nitrogen-containing compound, granulating the mixture by further continuing the reaction and thereafter dehydrating/drying the mixture. However, this phenol resin powder has an extremely large average particle diameter of about 1 mm. Further, the resin contains about 5.1% of free phenol, and is to be improved in view of safety.
Japanese Patent Laying-Open No. 58-74711 (Patent Document 4) describes a method for producing solid resol-type phenol resin powder by obtaining an initial condensation product by heating a mixture of a phenolic compound, an aldehyde and a nitrogen-containing compound catalyst, thereafter granulating the same by adding a cellulosic compound or an aqueous polymer compound while maintaining the same in a prescribed temperature region and then keeping the reaction system acidic. However, the phenol resin particles have a large average diameter of 200 μm to 1 mm, and contain a large quantity of free phenol.
Japanese Patent Laying-Open No. 59-80437 (Patent Document 5) describes a method of dispersing a reaction condensate of an aldehyde compound and a phenol compound with a dispersant, solidifying the condensate by a method of cooling the same below the melting point or the like to granulate the condensate. However, resin particles obtained by this method have large particle diameters of about 75 to 400 μm, and the particle size distribution is also wide.
Japanese Patent Laying-Open No. 61-51019 (Patent Document 6) discloses microspherical resol resin particles covered with inorganic salts insoluble in water and a production method of rendering the inorganic salts insoluble in water coexistent in condensation reaction. However, the microspherical resol resin particles have a large average particle diameter of about 40 to 80 μm. Further, the resin particles have such a problem that a rate of dispersion in water is slow when the particles covered with the inorganic salts insoluble in water are used as a water dispersion liquid.
Japanese Patent Laying-Open No. 2004-149656 (Patent Document 7) describes a spherical rosin-modified phenol resin having an average particle diameter of 0.1 μm to 10 μm and a method for producing a spherical rosin-modified phenol resin by emulsifying a rosin-modified phenol resin dissolved in an organic solvent with water and an emulsifier and thereafter performing solid-liquid separation. However, this spherical rosin-modified phenol resin has such problems that the strength and heat resistance of the phenol resin are insufficient due to the rosin-modification of the resin and the residual carbon ratio is so low that the use is restricted to a specific use such as a binder for printing ink or the like. As to the production method, further, such points are problematic that an aromatic solvent is employed and the application is restricted to a rosin-modified resin having a self-emulsifying function.
Patent Document 1: Japanese Patent Laying-Open No. 58-111822
Patent Document 2: Japanese Patent Laying-Open No. 2-167327
Patent Document 3: Japanese Patent Laying-Open No. 50-98537
Patent Document 4: Japanese Patent Laying-Open No. 58-74711
Patent Document 5: Japanese Patent Laying-Open No. 59-80437
Patent Document 6: Japanese Patent Laying-Open No. 61-51019
Patent Document 7: Japanese Patent Laying-Open No. 2004-149656